1. Field of the Invention
The present invention is generally related to extreme ultraviolet (EUV) photolithography systems, and more particularly, to measuring wavefront parameters in a photolithographic system.
2. Related Art
Lithography is a process used to create features on the surface of substrates. Such substrates can include those used in the manufacture of flat panel displays, circuit boards, various integrated circuits, and the like. A frequently used substrate for such applications is a semiconductor wafer. One skilled in the relevant art would recognize that the description herein would also apply to other types of substrates.
During lithography, a wafer, which is disposed on a wafer stage (WS), is exposed to an image projected onto the surface of the wafer by an exposure system located within a lithography system. The exposure system includes a reticle (also called a mask) for projecting the image onto the wafer.
The reticle is generally located between a semiconductor chip and a light source, usually mounted on a reticle stage (RS). In photolithography, the reticle is used as a photo mask for printing a circuit on a semiconductor chip, for example. Lithography light shines through the mask and then through a series of optical lenses that shrink the image. This small image is then projected onto the silicon or semiconductor wafer. The process is similar to how a camera bends light to form an image on film. The light plays an integral role in the lithographic process. For example, in the manufacture of microprocessors (also known as computer chips), the key to creating more powerful microprocessors is the size of the light's wavelength. The shorter the wavelength, the more transistors can be etched onto the silicon wafer. A silicon wafer with many transistors results in a more powerful, faster microprocessor.
As chip manufacturers have been able to use shorter wavelengths of light, they have encountered a problem of the shorter wavelength light becoming absorbed by the glass lenses that are intended to focus the light. Due to the absorption of the shorter wavelength light, the light fails to reach the silicon wafer. As a result, no circuit pattern is created on the silicon wafer. In an attempt to overcome this problem, chip manufacturers developed a lithography process known as Extreme Ultraviolet Lithography (EUVL). In this process, a glass lens can be replaced by a mirror.
The problem of measuring the quality of the illumination beam is a persistent one for the lithographic applications. In particular, it is desirable to be able to measure wavefront quality in the photolithographic tool during wafer production and exposure, rather than having to disassemble the tool in order to do so. The particular environmental needs of an EUV Tool make disassembly a long and arduous task.